1. Field of the Invention
The present invention relates to an improved process for the preparation of N-methylol-caprolactam through reaction of caprolactam with formaldehyde in the presence of an alkaline catalyst at elevated temperature.
2. Description of the Related Art
It has already been disclosed that N-methylolcaprolactam can be prepared in a yield of 67% of the theoretical yield from caprolactam and excess paraformaldehyde (50% excess) in 95% strength ethanol in the presence of 4.2 mol % of sodium hydroxide as catalyst (J. Am. Chem. Soc. 70 (1948), 2115-2118). The reaction specified is carried out in approximately 50% strength solution.
The procedure mentioned for the preparation of N-methylol-caprolactam is also in Houben-Weyl, Methoden der organischen Chemie [Methods of Organic Chemistry], 4th edition, vol. XI/2/1958) p. 570, but it is stated here that it was not possible to reproduce the procedure cited, and a yield of only 50% of the theoretical yield was achieved. A very similar procedure is furthermore given in DE-OS (German Published Specification) No. 2,616,374, likewise in only 50% of the theoretical yield. A short note on the preparation of N-methylolcaprolactam as a simple intermediate in the preparation of further caprolactam derivatives is furthermore found in Arch. Pharm. 294 (1961), pp. 344-348. However, only the starting materials caprolactam and paraformaldehyde are mentioned in this note; further notes on the reaction parameters are omitted. The N-methylol-caprolactam is obtained as an oil in this procedure; there is no mention of characterization and the yield of the N-methylol-caprolactam prepared.
U.S. Pat. No. 3,073,843 furthermore gives a preparation procedure for a formally similar compound, namely N-methylol-pyrrolidone, in which procedure potassium hydroxide was added to pyrrolidone, whereupon a suspension of potassium pyrrolidone is formed in the remaining pyrrolidone. Paraformaldehyde is then added, which leads to a spontaneous increase in temperature to 80.degree. C. The yield is given as essentially quantitative. However, this process cannot be applied to caprolactam since
(a) in contrast to caprolactam, 2-pyrrolidone becomes liquid at an only slightly elevated temperature;
(b) the 5-membered ring system of 2-pyrrolidone and 7-membered ring system of caprolactam have different reactivities. Thus, the reaction mixture according to U.S. Pat. No. 3,073,843 warms spontaneously to about 80.degree. C., whereas the mixture of caprolactam, paraformaldehyde and potassium hydroxide has to be melted initially by supplying energy and then brought to a suitable reaction temperature and kept there while supplying further energy;
(c) N-methylol-caprolactam is thermally labile, whereas, in contrast to this, N-methylol-pyrrolidone is a stable compound.